Add new instant extended specific pref.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 952b040e8c7d2ed284edbccdd64ee58c5530f27d..80aa9424e8e1b8e13cf8af1bf46bffac35f63adb 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -487,6 +487,9 @@ const char kInstantConfirmDialogShown[] = "instant.confirm_dialog_shown";
 // Boolean pref indicating if instant is enabled.
 const char kInstantEnabled[] = "instant.enabled";
 
+// Boolean pref indicating if instant is enabled for instant extended.
+const char kInstantExtendedEnabled[] = "instant_extended.enabled";
+
 // Prefix URL for the experimental Instant ZeroSuggest provider.
 const char kInstantUIZeroSuggestUrlPrefix[] =
     "instant_ui.zero_suggest_url_prefix";